<div style="
   position: absolute;
   top: 0px;
   left: 180px;
   min-width: 160px;
   height: 34px;
   overflow: hidden;
   background: #F2F2F2;
       -moz-border-radius:10px;
    -webkit-border-radius:10px;
    border-radius:2px;">
    
<select id="eje_id" onchange="cambiarEje()" style="background: transparent;
   min-width: 160px;
   padding: 5px;
   font-size: 16px;
   border: 1px solid #ccc;
   height: 34px;
">
    <option value=""></option>
    <?php foreach($ejes as $eje): ?>
        <option value="<?php echo $eje->getId();?>"><?php echo $eje->getTitulo(); ?></option>
    <?php endforeach; ?>
</select>
</div>

<span id="div_tema">
</span>

<script>
    function cambiarEje(){
        var tema_s = $("#eje_id").val();
        if(tema_s !=""){
            callUrl('div_tema', '<?php echo url_for("tema/selectTema"); ?>?tema_id='+tema_s);
        }
        else{
            $("#div_tema").html("");
        }
    }
</script>